Method of converting corrugated pipe molding machine

ABSTRACT

A blow-molding machine for molding corrugated plastic pipe having uninterrupted annular ribs thereon is converted for forming corrugated plastic pipe having interrupting gaps or recesses in certain annular ribs of corrugated pipe being formed. Essentially the conversion is effected by positioning and securing relatively short inserts or segmental blocks in otherwise uninterrupted semicircular valleys of cooperating pairs of semitubular mold sections of dies between and against which the plastic pipe is molded.

United States Patent [191 Maroschak [451 Feb. 4, 1975 METHOD OF CONVERTING CORRUGATED PIPE MOLDING MACHINE [75] Inventor: ErnestJ.Mar0schak,Roseboro,

[73] Assignee: Plastic Tubing, Inc., Roseboro, N.C.

[22] Filed: Dec. 10, 1973 21 Appl. No.: 422,993

Related U.S. Application Data [62] Division of Ser. No. 261,780, June 12, 1972, Pat. No. 3,784,346.

[52] U.S. Cl 264/95, 264/99, 264/151,

3,380,121 4/1968 Chittenden et al. 425/DIG. 210 3,430,292 3/1969 Bauman et a1 425/183 3,550,197 12/1970 Szajna et al 264/94 3,732,046 5/1973 Martin 264/40 I Primary Examiner-Jeffery R. Thurlow Attorney, Agent, or FirmParrott, Bell, Seltzer, Park & Gibson [57] ABSTRACT A blow-molding machine for molding corrugated plastic pipe having uninterrupted annular ribs thereon is converted for forming corrugated plastic pipe having interrupting gaps or recesses in certain annular ribs of corrugated pipe being formed. Essentially the conversion is effected by positioning and securing relatively short inserts or segmental blocks in otherwise uninterrupted semicircular valleys of cooperating pairs of semitubular mold sections of dies between and against [56] References Cited which the plastic pipe is molded.

UNITED STATES PATENTS 3 188 690 6/1965 Zieg 425/327 12 Clams l3 Drawmg figures "T ITTTTTWTITTTTTTT I, 36: 35 L7 I I I DEALLING CUTTING- EXTBUDWG- Hones To LENGTH I f 26 BL M k ow OLDING" 30 sum 10F 2 PATENTEU 4|975 METHOD OF CONVERTING CORRUGATED PIPE MOLDING MACHINE This application is a division of my copending application, Ser. No. 261,780, filed June 12, 1972 now Pat. No. 3,784,346, and entitled CONVERTED CORRU- GATED PIPE MOLDING MACHINE.

This invention relates to machinery for forming corrugated plastic pipes from a plastic material being continuously extruded into sucessive pairs of cooperating semitubular mold sections or dies having alternating transverse annular ribs and valleys internally thereof against which the extruded tube is expanded by compressed fluid to form respective valleys and ribs resulting in the corrugated plastic pipe. The invention is particularly concerned with converting such machinery for the production of corrugated plastic pipe having interrupting gaps or recesses in certain of the exterior ribs of the pipe.

In machinery for blow-molding extruded plastic-tubing to form corrugated pipe therefrom, it is necessary to use a substantial number of pairs of cooperating semi-tubular mold sections arranged end to end on two endless paths so that the pairs of mold sections are .sucessively advanced into an elongate molding zone and into substantially air-tight contiguous relation in alignment with the extruder. A typical blow-molding machine of this type is disclosed, for example, in U.S. Pat. No. 3,188,690, issued June 15, 1965. It is apparent that mold sections of the type described must be manufactured very precisely and within close tolerances Also, it is necessary to utilize from 35 to 50 or more pairs of cooperating'semitubular mold sections with a particular extruder in order to obtain a reasonably economical rate of production of continuous corrugated plastic pipe, since each mold section is normally of relatively short length of about four inches in order that they will properly traverse the endless tracks of a conventional molding machine. It can be appreciated therefore that considerable expense would be involved if it became necessary to replace many if not all of the mold sections in order to change the exterior configuration of the continuous corrugated plastic pipe to be produced on a particular blow-molding machine.

The conventional substantially semitubular mold sections utilized in the blow-molding of'corrugated plastic pipes are each provided with substantially semicicular transverse internal alternating ribs and valleys whereby the ribs and valleys of one section of each pair cooperate with the ribs and valleys of the other section of the same pair to form uninterrupted annular ribs and valleys while the sections advance in the molding zoneto thus form corrugated plastic pipe having uninterrupted annular ribs and valleys.

Although corrugated plastic pipe with uninterrupted annular ribs has many uses in that form, the laws of many states require that corrugated pipe used for septic tank drainage fields be at least about 4 inches in diameter and that sets of angularly spaced drainage holes be provided in the wall of the corrugated pipe at certain longitudinally spaced locations. Further,-su'ch laws require that the portions of the walls of the corrugated pipe through which the drainage holes extend are shielded to some degree to aid in preventing these holes from becoming clogged with earth in which the corrugated pipe is buried. Therefore, it is desirable to mold the corrugated pipe with interrupting recesses or gaps in certain of the annular ribs. Corrugated drainage pipe of this type is disclosed and claimed in my commonly assigned and copending application Ser. No. 228,508, filed Feb. 23, 1972, now U.S. Pat. No. 3,747,352.

It is therefore an object of this invention to provide a simple and economical method of converting a blowmolding machine for forming continuous corrugated plastic pipe so as to form interrupting gaps or recesses in certain annular ribs of the plastic pipe being formed between adjacent pairs of cooperating semitubular mold sections of the character heretofore described.

'It is a more specific object to provide a method of converting a blow-molding machine of the type described, wherein at least one insert is positioned in one of the valleys of at least one of the mold sections of a pair with the insert being of an arcuate length substantially less than that of the respective valley so that molding of the plastic pipe against the insert forms an interrupting gap in the corresponding ribs being molded.

Another object is to provide a method of 'converting a blow-molding machine of the type described wherein at least one insert is positioned in one of the valleys in at least one of the mold sections of a cooperating pair and releasably secured in the respective valley so that, when the molding machine is not to be used for forming corrugated plastic pipe with gaps in some of the annular ribs thereof, the insert may be removed from the corresponding mold section to reconvert the same back of its original form, or the mold section may be readily converted to another form if a change is desired in the shape or number of interrupting gaps to be present in the corrugated pipe being molded.

Some of the objects of the invention having been stated, other objects will appear as the description proceeds when taken in connection with the accompanying drawings, in which FIG. 1 is a schematic plan view of a typical blowmolding machine for forming corrugated plastic pipe;

FIG. 2 is a perspective view of a conventional pair of cooperating semitubular mold sections removed from the blow-molding machine of FIG. 1, but showing the two mold sections in exploded relationship to illustrate the uninterupted nature of the substantially semicircular transverse internal valleys between the ribs of each mold section;

FIG. 3 is a fragmentary view of a length of the corrugated plastic pipe produced utilizing the conventional mold sections of the type shown in FIG. 2',

FIG. 4 is a schematic perspective view similar to FIG. 2, but showing one embodiment of the invention in which one of the mold sections of the corresponding pair has a plurality of spaced apart inserts positioned in one of the substantially semicircular valleys thereof;

FIG. 5 is a view similar to FIG. 3, but showing the corrugated plastic pipe as produced utilizing mold sections modified as shown in FIG. 4, and particularly illustrating the interrupting recesses or gaps in certain spaced annular ribs of the corrugated plastic pipe;

FIG. 6 is an enlarged view of the right-hand portion of the length of corrugated pipe shown in FIG. 5 after holes have been drilled in the wall portions of the corrugated pipe between the proximal ends of the interrupted rib segments; I

FIG. 7 is a transverse sectional, view taken substantially along line 7-7 in FIG. 6;

t is about 4 inches. The ribs and valleys 30, 31

FIG. 8 is an enlarged partially exploded view of the right-hand mold section shown in FIG. 4 so as to better illustrate the spaced apart inserts;

FIG. 9 is a transverse vertical sectional view through the pair of cooperating semitubular mold sections shown in FIG. 4 to illustrate the contiguous relation therebetween as they are advanced along the molding zone;

FIG. 10 is a fragmentary sectional plan view taken substantially along line 1010 in FIG. 9, but wherein the pair of semitubular mold sections are turned to accord with the position occupied thereby in their movement along the molding zone from left to right in FIG. 1;

FIG. 11 is a view similar to FIG. 9, but showing a second embodiment of the invention wherein inserts are positioned in both the mold sections of the corresponding pair, with IV: inserts positioned in each of the two mold sections;

FIG. 12 is a fragmentary vertical sectional view similar to the right-hand portion of FIG. 9, but showing a further embodiment of the invention, wherein the inserts are each of substantially less thickness than the depth of the corresponding valley in which they are positioned; and

FIG. 13 is a view similar to the lower portion of FIG. 10, but being taken substantially along line l3--l3 in .FIG.,12.

Referring more specifically to the drawings, and especially to FIG. 1, there is shown in typical blowmolding machine of conventional or other construction which may be of the general type disclosed in said US. Pat. No. 3,188,690, to which reference is made for a more detailed disclosure. The molding machine is of a type generally shown as a revolving section molding machine and is adapted to produce a continuous length of corrugated plastic pipe having alternating peripheral ribs and valleys 21, 22 along its length, the ribs 21 being equally spacedand defining the valleys 22 therebetween.

A plurality of pairs of cooperating substantially semitubular mold sections 24 are arranged to move through an elongate molding zone generally designated at 25.

Each pair of cooperating mold sections 24 includes a first or right-hand mold section 26 and a second or lefthand mold section 27. The first mold sections 26 are arranged in end-to-end relation, and the second mold sections 27 are also arranged'in end-to-end relation, but the first and second mold sections 26, 27 move in two endless paths determined by usual guide rails, not shown. Suitable drive means, not shown, is associated with the aforementioned guide rails for advancing the mold sections-26, 27 at identical speeds so that the mold sections 26, 27 of each cooperating pair 24 are I then in precise alignment and in contiguous relation as they enter the molding zone and move forwardly, from left to right, in FIG. 1. Conventionally, as shown in FIG. 2, each mold section has substantially semicircular transverse internal alternating uninterrupted ribs and valleys 30, 31 along the axial length thereof, there being about six ribs 30 and six valleys 31 defining a generally semicircular mold cavity in each of the first and second semitubular mold sections 26, 27 of each cooperating pair24. The axial length of each mold section of the first mold section 26 of each pair 24 cooperate with the ribs and valleys 30, 31 of the other mold section 27 of the same pair so as to form uninterrupted annular ribs and valleys as the sections 26, 27 advance forwardly along molding zone 25.-

As mold sections 26, 27 advance and enter molding zone 25, they enclose around the extended mandrel or nozzle 35 of an extruder 36 which is continuously extruding an annular tube of molten plastic material forwardly into molding zone 25. As is conventional, as the cooperating pairs of mold sections 24 advance along molding zone 25 forwardly of nozzle 35 of extruder 36, compressed air of other fluid is introduced into the plastic tube to cause the same to expand and conform substantially to the annular ribs and valleys defined by the cooperating pairs of mold sections 24. Thus, the alternating semicircular internal transverse ribs and valleys in each pair of mold sections 24 form respective uninterrupted annular valleys and ribs on the plastic pipe being continuously extruded into the successivepairs of mold sections 24 entering and advancing along molding zone 25.

As shown in the right-hand portion of FIG. 1, suitable means D are provided for drilling holes in the corrugated plastic pipe and cutting means C is provided for then cutting the pipe 20 to predetermined lengths as is desirable.

As heretofore stated, it is desirable to have the wall portions of the corrugated plastic pipe through which drainage holes areto be formed located so that they are shielded to some degree by adjacent rib portions of the corrugated plastic pipe. Therefore, instead of forming the corrugated pipe as in FIG. 3, wherein all the ribs and valleys throughout the length of the pipe are uninterrupted, it is desirable to provide a corrugated plastic pipe of the type indicated at 20a in FIG. 5, wherein at longitudinally spaced intervals, which are preferably about four inches apart, interrupting gaps or recesses are provided in corresponding annular ribs of the plastic pipe so that, in effect, interconnecting lands are defined between the proximal ends of adjacent segments of the corresponding ribs.

More specifically, it will be observed in FIG. 5 that the corrugated plastic pipe 20a there shown includes alternating annular ribs and valleys 21a, 22a along its length with most of the ribs 21a being uninterrupted and the valleys therebetween being identified at 22a. However, every sixth rib 21a, in this instance, is formed in segments and has three interrupting gaps or recesses a, b, c therein whose lands or peripheral surfaces are substantially flush with the bottoms of the valleys 22a of the corrugated plastic pipe 20a.

In order to produce the form of corrugated plastic pipe 20a shown in FIG. 5 according to the first embodiment of the invention shown in FIGS. 4, 8, 9 and 10, a plurality of inserts or segmental blocks, preferably formed of metal, are inserted in one valley 31 adjacent the central portion of one of the mold sections of each pair. In this instance, three relatively short and substantially equally spaced inserts 41, 42, 43 are inserted in a valley 3] in a medial portion of the first mold section 26 shown in FIGS. 4, 8, 9 and 10. It is preferred that all of the inserts 41, 42 43 are of about the same length.

Of course, the inserts are formed before they are inserted in the corresponding valley 31 and, as shown, each insert 41, 42, 43 is beveled at an angle of about 45 at each end thereof and is of an arcuate length substantially less than that of the respective valley 31 in which it is to be inserted. In this particular embodiment, each insert 41, 42, 43 is a thickness about equal to the depth of the respective valley 31. When installed, it is preferred that the centers of the adjacent inserts 41, 42, 43 are spaced about 60 apart.

In order that the inserts 41, 42, 43 are properly retained in the desired predetermined positions in the desired valley 3!, but may be readily removed and/or replaced without seriously damaging or mutilating the corresponding mold section or mold sections, any suitable means may be used for releasably securing the inserts in the desired mold section or mold sections. Preferably, the inserts 41, 42, 43 are releasably secured in the corresponding valley 3! by means of a fusible metal having a relatively low melting point, such as so-called cold solder; i.e., a mixture of about one part lead to one part zinc. Obviously, the solder cannot be seen in the drawings, but it may be positioned along the radially outward surface and opposite sides of each insert with each insert being positioned against the bottom and adjacent side surfaces of the corresponding valley 31. Since the mold sections 26, 27 are normally formed of hardened steel, the heat required for fusing the inserts to or releasing them from a mold section will not adversely affect the mold section to any substantial degree.

It is thus seen that, by providing each ofthe first mold sections 26 with a set of three inserts 41, 42, 43 arranged in the manner heretofore described, every sixth rib 21a in the corrugated plastic pipe 20a being formed will be provided with the three interrupting gaps a, b, c therein as best shown in FIG. 7. In this regard, the drilling of holes a, b',c in the lands formed between'the adjacent segments of each interrupted rib 21a may be effected at drilling station D downstream of molding zone 25, and the corrugated plastic pipe thus formed may be cut into the desired lengths at cutting station C in FIG. I concurrently with the molding of the corrugated plastic pipe upstream of the stations C, D.

The second embodiment of the invention shown in FIG. 11 differs from the-first embodiment shown in FIGS. 4,8, 9 and 10 in that, instead of all three of the inserts or segmental blocks being placed in one of the mold sections of a cooperating pair 24, they are positioned in both mold sections of the same pair. More specifically, it will be observed in FIG. 11 that two distal inserts 41, 43 are positioned entirely in the respective first and second mold sections 26, 27, but the intermediate insert 42, spaced between the distal inserts 41', 43' is formed in two pieces or parts of approximately equal length which abut each other at the juncture of the lower portions of the two contiguous mold sections 26, 27 of the corresponding pair 24.

The combined length of the two parts of the intermediate insert 42' in FIG. II is preferably about equal to the total length of the distal inserts 41', 43'. Thus, in effect, the aligned semicircular valleys 31 in the two mold sections 26, 27 of a cooperating pair 24 each have lVzinserts therein in FIG 11. It is apparent that, by providing all of the mold section. pairs 24 with a set of the inserts 41, 42, 43 arranged as shown in FIG. 11, the corrugated plastic pipe being produced would correspond with the plastic pipe a shown in FIG. 5. Thus, since each mold section pair 24 is about 4 inches long, an interrupted rib 21a would be molded about every 4 inches along pipe 20a.

It is desirable in most instances to provide a relatively I shallow rib portion in each of the interrupting gaps being formed in certain annular ribs of the plastic pipe in accordance with this invention so that, in effect, a plateau of intermediate height and of about the same width as the adjacent rib will extend between the proximal ends of adjacent segments of the corresponding annular ribs having the interrupting gaps therein. Accordingly. the third form of the invention is' provided as shown in FIGS. 12 and 13, wherein only the first mold section 26 is illustrated. It will be observed that three spaced apart inserts 41", 42", 43 also are secured in one of the valleys 3] of the first mold section 26 shown in FIGS. 12 and 13. However, each of the inserts 41". 42", 43 is of less thickness than the depth of the respective semicircular valley 31 so that the innermost surfaces of the inserts 41", 42", 43" closest to the axis of the semitubular mold section 26 are spaced radially voutwardly of the innermost semicircular surfaces of the adjacent semicircular ribs 30.

To lend strength to the rib segments being molded in the corrugated pipe, the inserts 41", 42", 43" are formed with substantially perpendicular ends before they are positioned in and secured in the corresponding valley. Thus, the opposing ends of the latter'inserts extend in respective substantially radial planes with respect to the axis of'the ribs and valleys of the corresponding pair of mold sections.

Although three spaced apart inserts are described herein as being positioned, in each pair of cooperating mold sections 24, it is apparent that only one or two inserts may be used in either or both mold sections 26,

27 of a cooperating pair 24, without departing from the invention. Also, it is to be understood that, in instances where it is desired to mold a bell portion, coupler or non-corrugated portion at spaced intervals along and integral with the continuous corrugated plastic pipe being molded, the adjacent cooperating pair or pairs of mold sections would not have any inserts therein corresponding to those inserts 41-43, 4l-43 or 41"-4'3" described earlier herein. However, in such event, at least the majority of the cooperating pairs of semitubu-. Iar mold sections still would each have at least one insert in one of the valleys thereof. Therefore, as preferred, one annular valley of each of at least a majority of the cooperating pairs'of mold sections 24 would have three spaced apart inserts secured therein. Reference is made to my commonly assigned and copending application Ser. No. 254,934, filed May 19, I972, and entitled CORRUGATED PLASTIC PIPE WITH INTE- GRALLY FORMED COUPLER, for a detailed discIo-.

sure of an integral bell or coupler on one end of a length of corrugated resilient plastic pipe.

It is thus seen that I have provided a converted corrugated pipe molding machine and a method of obtaining such conversion in an economical manner so as to form a variety of different forms of corrugated pipes.

In the drawings and specification there have been set forth preferred embodiments of the invention and although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation.

That which is claimed is:

1. In a method of molding plastic pipe wherein an annular tube of molten plastic material is extruded and directed into a molding machine having pairs of cooperating mold sections arranged to receive the extruded tube and move the same through an elongate molding zone and wherein at least some of the mold sections have alternating semicircular transverse ribs and valleys with the ribs and valleys of one section of each pair cooperating with the ribs and valleys of the other section of the same pair to form respective annular valleys and ribs on the plastic pipe being molded therein, the improvement comprising positioning and securing at least one insert in one of the valleys of at least one of the mold sections and thereafter molding plastic pipe against the mold section whereby the insert serves for forming an interrupting gap in the corresponding rib being molded.

2. A method according to claim 1, which includes positioning and securing at least one additional insert in said one of the valleys in spaced relation from said first named insert with the combined length of both inserts being substantially less than that of the respective valley, whereby at least two interrupting gaps are formed in the same rib during molding of the plastic pipe.

3. A method according to claim 1, which includes forming the insert of a thickness substantially less than the depth of the respective valley before positioning and securing the insert in the respective valley.

4. A method according to claim 1, which includes forming the insert of a thickness substantially equal to the depth of the respective valley before positioning and securing the insert in the respective valley.

5. A method according to claim l,- which includesforming substantially perpendicular ends on the insert before positioning and securing the same in the respective valley.

6. A method according to claim 1, which includes forming beveled opposite ends on the insert before positioning and securing the same in the respective valley.

7. A method according to claim 1, which includes positioning and securing an additional insert in the coop erating valley'of the other mold section of the same pair so that both inserts are in the same annular valley.

8. A method according to claim 1, which includes positioning and securing at least one insert in one of the valleys of one mold section of each of a majority of the pairs.

9. A method according to claim 1, which includes positioning and securing three spaced apart inserts in one of the valleys of one mold section of each a majority of the pairs.

10. A method according to claim 1, which includes positioning and securing three spaced apart inserts in one of the annular valleys of each cooperating pair of mold sections.

11. A method of according to claim 1, wherein the step of securing the insert in the valley of the mold section'comprises releasably securing the same to readily permit later reconverting the mold section back to its original form.

12. In a method of molding plastic pipe wherein an annular tube of molten plastic material is extruded and directed into a molding machine having pairs of cooperating mold sections arranged to receive the extruded tube and move the same through an elongate molding zone and wherein at least some of the mold sections have alternating semicircular transverse ribs and valleys will the ribs and valleys of one section of each pair cooperating with the ribs and vallesy of the other section of the same pair to form respective annular valleys and ribs on the plastic pipe being molded therein, the

improvement comprising forming a plurality of inserts, each of a width corresponding to that of the valleys of the mold section and of an arcuate length substantially less than that of the valleys and of a thickness not exceeding the depth of the valleys, positioning and securing the inserts in spaced apart relation in one of the valleys of at least-one of the mold sections and thereafter molding plastic pipe against the mold section whereby the inserts serve for forming interrupting gaps in the corresponding rib being molded.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 864, 446 DATED February 4, 1975 |NVENTOR(5) Ernest J. Maroschak It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, Line 24, "sucessively" should be --successlvely-; Line 46, "semicicular" should be "semicircular". Column 2, Line 30, "of" should be --to--. Column 3, Line 30, "in" should be -a-. Column 4, Line 11, "of" should be -or-. Column 5, Line 57, after "of" insert --each of--. Column 8, CLAIM 12, Line 25, "will" should be --with--; Line 26, "vallesy" should be --va.lleys--.

Signed and sealed this 29th day of April 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks 

1. In a method of molding plastic pipe wherein an annular tube of molten plastic material is extruded and directed into a molding machine having pairs of cooperating mold sections arranged to receive the extruded tube and move the same through an elongate molding zone and wherein at least some of the mold sections have alternating semicircular transverse ribs and valleys with the ribs and valleys of one section of each pair cooperating with the ribs and valleys of the other section of the same pair to form respective annular valleys and ribs on the plastic pipe being molded therein, the improvement comprising positioning and securing at least one insert in one of the valleys of at least one of the mold sections and thereafter molding plastic pipe against the mold section whereby the insert serves for forming an interrupting gap in the corresponding rib being molded.
 2. A method according to claim 1, which includes positioning and securing at least one additional insert in said one of the valleys in spaced relation from said first named insert with the combined length of both inserts being substantially less than that of the respective valley, whereby at least two interrupting gaps are formed in the same rib during molding of the plastic pipe.
 3. A method according to claim 1, which includes forming the insert of a thickness substantially less than the depth of the respective valley before positioning and securing the insert in the respective valley.
 4. A method according to claim 1, which includes forming the insert of a thickness substantially equal to the depth of the respective valley before positioning and securing the insert in the respective valley.
 5. A method according to claim 1, which includes forming substantially perpendicular ends on the insert before positioning and securing the same in the respective valley.
 6. A method according to claim 1, which includes forming beveled opposite ends on the insert before positioning and securing the same in the respective valley.
 7. A method according to claim 1, which includes positioning and securing an additional insert in the cooperating valley of the other mold section of the same pair so that both inserts are in the same annular valley.
 8. A method according to claim 1, which includes positioning and securing at least one insert in one of the valleys of one mold section of each of a majority of the pairs.
 9. A method according to claim 1, which includes positioning and securing three spaced apart inserts in one of the valleys of one mold section of each a majority of the pairs.
 10. A method according to claim 1, which includes positioning and securing three spaced apart inserts in one of the annular valleys of each cooperating pair of mold sections.
 11. A method of according to claim 1, wherein the step of securing the insert in the valley of the mold section comprises releasably securing the same to readily permit later reconverting the mold section back to its original form.
 12. In a method of molding plastic pipe wherein an annular tube of molten plastic material is extruded and directed into a molding machine having pairs of cooperating mold sections arranged to receive the extruded tube and move the same through an elongate molding zone and wherein at least some of the mold sections have alternating semicircular transverse ribs and valleys will the ribs and valleys of one section of each pair cooperating with the ribs and vallesy of the other section of the same pair to form respective annular valleys and ribs on the plastic pipe being molded therein, the improvement comprising forming a plurality of inserts, each of a width corresponding to that of the valleys of the mold section and of an arcuate length substantially less than that of the valleys and of a thickness not exceeding the depth of the valleys, positioning and securing the inserts in spaced apart relation in one of the valleys of at least one of the mold sections and thereafter molding plastic pipe against the mold section whereby the inserts serve for forming interrupting gaps in the corresponding rib being molded. 